1. Field of the Invention
The present invention relates to an apparatus for converting video signals, and in particular, to an apparatus for converting field video signals into frame video signals.
2. Description of the Prior Art
It has been well known in the art that interlaced scanning is utilized for video signals such as television signals undergoing raster scanning. In the standard color television signal system of NTSC, for example, an interlaced scanning is conducted to form a frame including a couple of fields.
When recording video signals on a rotating recording medium such as a magnetic disk, a recording scheme, that is, so-called "field recording" is used in some cases in which either one of an odd-numbered field or even-numbered field is recorded on a track of the rotating recording medium. In order to reproduce such field video signals obtained by the field recording and to display the reproduced signals on a video monitor screen, the field video signals must be converted into frame video signals in conformity with the standard signal format.
In a playback system in which video signals are repetitiously read from the same field on a disk track, such field video signals are successively read on a time-serial basis. On the other hand, the interlaced scanning is executed in the standard signal system; consequently, in order to generate two interlaced fields constituting a frame from video signals recorded in the same field, the video signals in an even-numbered field must have a delay, a period of which is equivalent to one half a horizontal scanning period (1H) with respect to those in an associated odd-numbered field.
In more detail, referring to FIG. 1, horizontal scanning lines of odd-numbered and even-numbered fields are depicted with solid and dotted lines, respectively. As can be understood from these lines, if a frame comprises 525 horizontal scanning lines, for example, the odd-numbered field terminates at the center of 263rd scanning line #263H, that is, 262.5H, which is followed by the even-numbered field. To generate a proper picture with the even-numbered field arranged in the horizontal scanning line #264 beginning from the top thereof, the contents of video signals written in #264H must correctly correspond to those of video signals recorded in #1H. Field video signals supplied from a magnetic disk return from position 262.5H to position 0.5H which is the top of horizontal line #1H. To obtain such an appropriate correspondence, the video signals in the even-numbered field must be delayed with respect to those in the odd-numbered field by a time period substantially equal to one half of a horizontal scanning period, 1H.
Field video signals obtained from a magnetic disk are ordinarily separated into luminance (Y) signals and chroma (C) signals according to the line sequential color television system; which are then demodulated. Therefore, the 0.5H delay and the changeover operation for each field must be conducted for both Y and C signals.
Above-mentioned operations require two delay circuit systems and two changeover circuit systems, so complex circuit configuration is unavoidable; furthermore, difference in circuit gain between a signal passed through a 0.5H delay circuit and a signal not passed therethrough causes flicker in the reproduced picture at a frequency equal to one half the vertical scanning frequency fv. Consequently, a complex, difficult circuit adjustment is necessitated to remove such a flicker from the picture.
To avoid the difficulty in adjusting circuits to prevent the flicker, it is only necessary to arrange a delay circuit and a changeover circuit on the output section to which a composite color video signal is delivered. That is, color difference signals, such as R-Y and B-Y signals are generated from the chroma signal in the line sequential color television system to conduct the balanced modulation on the chrominance subcarrier, and the resultant signal is then combined with the luminance signal. Afterwards, the thus obtained composite signal need only be delivered to a 0.5H delay circuit and a odd-numbered/even-numbered field changeover circuit.
In such a circuit configuration, however, if the signals which do not pass the delay circuit are recorded in odd-numbered fields, the phase of chrominance signal written in an even-numbered field is advanced by 90.degree. with respect to that in the associated odd-numbered field. In an automatic phase control circuit of the video playback monitor, the synchronization of color signals (color lock) is lost at the beginning of each field. Therefore, the hue is changed so that a false color appears at the top portion of a reproduced image of each field. This phenomenon is magnified and is clearly developed in a hardcopy image such as a printed picture, thereby bringing forth a disadvantageous feature in practical use. In a monochrome video monitor, the chrominance signals are generally not removed from the luminance signals. When video signals including chrominance signals whose phase is shifted as described above (which causes the charge in hue) are reproduced on such a monochrome video monitor, fluctuation in the luminance due to chrominance signals is not cancelled for each frame. Consequently, dots having brightness associated with the waveforms of respective chrominance signals appear in a reproduced picture.